You are Here: Home >< Physics

# newton's law of gravitation confusion Watch

1. quoted from my text book:

F = GMm/r^2

"where r is the distance apart from their centres"

g = F/m = - GM/r^2

Using the equation above, calculate a value for the mass of the Earth (radius of Earth = 6.38 x 10^6m)

M = gr^2/G = 9.81 x (6.38 x 10^6)^2 / 6.67 x 10^-11

... i thought r represented the distance from the centre of two masses (NOT the radius of an object?)
2. Hello Maths guy

In this "g" is the acceleration due to gravity on the surface of earth.
now if you place a point particle of unit mass (m) on the surface of the earth (M), the distance between the two masses = radius of Earth. I hope this clarifies.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: December 8, 2010
Today on TSR

### Graduate sues Oxford for £1m

Claims damages because he didn't get a first

### Get ready! TSR's giving away free money

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.